Follow-up to r1546409: Fix fs module deinitialization such that we are able
to re-init it later.  This is required to make pass ruby binding tests pass.

* subversion/libsvn_fs/fs-loader.c
  (common_pool,
   common_pool_lock): Ensure that all global variables always have
                      well-defined values.
  (uninit): Mark everything as "not initialized" again.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1546723 13f79535-47bb-0310-9956-ffa450edef68